Objective lens unit, optical pickup, and optical information apparatus

ABSTRACT

An objective lens unit according to the present invention includes a first objective lens  41 ; and a first lens holder  2  for supporting the first objective lens  41 . The first lens holder  2  is formed of a material which transmits ultraviolet. Preferably, the first lens holder  2  includes a through-hole, having first and second openings  2   a  and  2   b , through which light incident on the first objective lens  41  passes, and an opening limiting section  3  provided along a circumferential direction of the through-hole and projecting toward a central axis of the through-hole. The first objective lens  41  is supported so as to block the first opening  2   b . The opening limiting section  3  guides light incident thereon from the second opening  2   a  in a direction away from an optical axis of the first objective lens.

This is a continuation application of U.S. application Ser. No.12/064,360 filed on Feb. 21, 2008 now U.S. Pat. No. 7,944,798, which isa §371 of International Application PCT/JP2006/316641, with aninternational filing date of Aug. 24, 2006 which claims priority to JPApplication Nos. 2005-245606 filed on Aug. 26, 2005, 2005-247106 filedon Aug. 29, 2005 and 2005-369638 filed on Dec. 22, 2005, the entirecontents of which are expressly incorporated by reference herein and isrelated to co-pending sibling U.S. Application filed on Jan. 26, 2011.

TECHNICAL FIELD

The present invention relates to an optical information apparatus foroptically performing recording or reproduction of information, and anoptical pickup and an objective lens unit usable for the opticalinformation apparatus. The present invention also relates to anapparatus having such an optical information apparatus applied thereto.

BACKGROUND ART

Optical discs are widely used as information recording mediums capableof recording a large capacity of information. Along with the progress ofthe technology, optical discs having a higher recording density havebeen developed.

The optical discs which were first commonly used are compact discs(CDs), and then digital versatile discs (DVDs) have become common. DVDscan record information at a recording density about six times therecording density of CDs. Since a large capacity of data can be recordedon one DVD, DVDs are especially used for recording video informationhaving a large information amount. Recently, optical discs capable ofrecording information at a still higher recording density, for example,HD-DVDs and Blu-ray discs (BDs), have been developed and have begun tobe used especially for recording high precision video information.

As various types of optical discs are developed, compatibility amongoptical disc apparatuses becomes important. In consideration of theconvenience for users, it is preferable that optical disc apparatusesare compatible to a plurality of types of optical discs.

As optical pickups for an optical disc apparatus compatible to aplurality of types of optical discs having different recordingdensities, optical pickups including a plurality of objective lenses areproposed as shown in, for example, Patent Documents 1 through 3. Suchconventional optical pickups are mainly compatible to CDs and DVDs.

Patent Document 1: Japanese Laid-Open Patent Publication No. 10-11765

Patent Document 2: Japanese Laid-Open Patent Publication No. 11-120587

Patent Document 3: Japanese Laid-Open Patent Publication No. 2002-245650

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

However, the present inventors examined optical disc apparatusescompatible both to an optical disc capable of recording information at ahigher recording density such as HD-DVDs or BDs and to conventional CDsand DVDs, and found the following problems.

For example, in an optical pickup including two objective lenses, thetwo objective lenses are supported by a common objective lens holder,and the objective lens holder is driven by one objective lens actuator.

Accordingly, the two objective lenses each need to be secured to theobjective lens holder while being optimally positioned to an opticalaxis in an optical system which uses the respective objective lens. Inorder to realize this, Patent Document 1 discloses the following. Oneobjective lens is secured to a tilting holder. The inclination of thetitling holder with respect to the objective lens holder is adjusted andsecured with an adjusting screw. Then, the titling holder is fixed tothe objective lens holder with an adhesive.

With this structure, however, the tilting holder needs to be securedonce with an adjusting screw. This makes the positional adjustmenttroublesome. There is another problem that a size of an opening for theobjective lens varies depending on the inclination of the tiltingholder, and so a desired numerical aperture (NA) cannot be guaranteed.

With an optical disc such as a HD-DVD or a BD, an optical spot forperforming recording or reproduction needs to be made small using anobjective lens having a large numerical aperture (NA) in order toachieve the high recording density. This requires a large through-holeto be formed in the lens holder for securing the objective lens.

Due to the large through-hole, such a lens holder has a reduced rigidityand thus is likely to be resonated at a predetermined frequency. As aresult of examinations, the present inventor found that this resonanceinfluences the operation of the objective lens actuator for driving theobjective lens holder, which may occasionally make it difficult tocontrol the objective lens at a high level of precision.

For performing a recording or reproduction operation on or from aplurality of optical discs having different recording densities, therequired control precision varies. In general, for performing arecording or reproduction operation on or from an optical disc having alow recording density, a high level of control precision is notrequired. However, due to the low recording density, the objective lensis required to move in a large area. By contrast, for performing arecording or reproduction operation on or from an optical disc having ahigh recording density, a high level of control precision is required,but the area in which the objective lens moves may be small.

In the case where a plurality of objective lenses are supported by anobjective lens holder, it is difficult to fulfill such differentspecifications because the objective lens holder is driven by oneactuator.

The present invention made to solve at least one of these problems ofthe conventional art has an object of providing an optical pickupincluding a plurality of objective lenses, and an optical informationapparatus.

Means for Solving the Problems

An objective lens unit according to the present invention comprises afirst objective lens; and a first lens holder for supporting the firstobjective lens. The first lens holder is formed of a material whichtransmits ultraviolet.

In one preferable embodiment, the first lens holder includes athrough-hole, having first and second openings, through which lightincident on the first objective lens passes, and an opening limitingsection provided along a circumferential direction of the through-holeand projecting toward a central axis of the through-hole. The firstobjective lens is supported so as to block the first opening. Theopening limiting section guides light incident thereon from the secondopening in a direction away from an optical axis of the first objectivelens.

In one preferable embodiment, the opening limiting section has a firstring-shaped inclined surface inclining with respect to an inner surfaceof the through-hole so as to face the first opening.

In one preferable embodiment, the opening limiting section has a secondring-shaped inclined surface inclining with respect to an inner surfaceof the through-hole so as to face the second opening.

In one preferable embodiment, the opening limiting section has a firstring-shaped inclined surface inclining with respect to an inner surfaceof the through-hole so as to face the first opening, and a secondring-shaped inclined surface inclining with respect to an inner surfaceof the through-hole so as to face the second opening.

In one preferable embodiment, where an angle made by light incident onthe second ring-shaped inclined surface and the normal to the secondring-shaped inclined surface is A1, an angle made by light output fromthe second ring-shaped inclined surface and the normal to the secondring-shaped inclined surface is A2, an angle made by the light incidenton the second ring-shaped inclined surface and the normal to the firstring-shaped inclined surface is A3, and a refractive index of theopening limiting section is n, the objective lens unit fulfillsrelationships of sin(A1)=n·sin(A2) and n·sin(A3+(A1−A2))>1.

In one preferable embodiment, the ring-shaped inclined surface has twodiscontinuous ring-shaped inclined surfaces located concentrically.

In one preferable embodiment, the opening limiting section has a shapeforming a part of a concave lens having an axis matching the centralaxis of the through-hole.

In one preferable embodiment, a cross-section of the opening limitingsection taken along the central axis of the through-hole is a part of anellipse projecting toward the central axis.

In one preferable embodiment, the opening limiting section has adiffraction grating provided so as to face the first opening.

In one preferable embodiment, the opening limiting section has ascattering surface provided so as to face the first opening forscattering light.

In one preferable embodiment, the opening limiting section has a lightbeam shielding surface provided so as to face the first opening forshielding light.

An optical pickup according to the present invention comprises a firstlight source; an objective lens defined by any one of the above; asupport for supporting the objective lens; an actuator for driving thesupport; and a first light detector. Light emitted by the light sourceis collected on a data recording face of an optical disc by the firstobjective lens of the objective lens, and light reflected by the datarecording face is converted into an electric signal by the lightdetector.

In one preferable embodiment, the first lens holder of the objectivelens unit is bonded to the support with an ultraviolet-curable resin.

In one preferable embodiment, the optical pickup further comprises asecond objective lens which does not share an optical axis with anoptical system formed by the objective lens unit, and a second lightsource. The support is a second lens holder for supporting the secondobjective lens.

In one preferable embodiment, an interval between an optical axis of thefirst objective lens and an optical axis of the second objective lens is5 mm or less.

In one preferable embodiment, an interval between an optical axis of thefirst objective lens and an optical axis of the second objective lens is2.5 mm or greater and 5 mm or less.

In one preferable embodiment, the first objective lens and the secondobjective lens are arranged in a tracking direction of the optical disc.

In one preferable embodiment, the first objective lens and the secondobjective lens are arranged in a direction perpendicular to a trackingdirection of the optical disc.

In one preferable embodiment, the first lens holder has a flat sidesurface at a position proximate to the second objective lens.

In one preferable embodiment, the first lens holder has a flat sidesurface at a position facing an outer peripheral area of the opticaldisc.

In one preferable embodiment, the first objective lens is used tocollect light having a longer wavelength than light collected by thesecond objective lens.

In one preferable embodiment, the first lens holder has a cutout at aposition symmetrical to the flat side surface with respect to an opticalaxis of the first objective lens.

In one preferable embodiment, the optical pickup further comprises aprojection projecting from each of the first lens holder and the secondlens holder more than the first objective lens and the second objectivelens. The projection is provided in an area other than an area facing anouter peripheral area of the optical disc.

In one preferable embodiment, the optical pickup further comprises asecond light source, a second detector and a second objective lens. Thefirst light source emits light having a first wavelength; the firstobjective lens collects the light emitted by the first light sourcetoward a recording face of a first optical disc; the first detectorreceives reflected light of the light collected on the recording face ofthe first optical disc and outputs a detection signal; the second lightsource emits light having a second wavelength which is shorter than thefirst wavelength; the second objective lens collects the light emittedby the second light source toward a recording face of a second opticaldisc; the second detector receives reflected light of the lightcollected on the recording face of the second optical disc and outputs adetection signal; and a focusing detection range of a focusing errorsignal generated based on the detection signal output by the firstdetector is larger than a focusing detection range of a focusing errorsignal generated based on the detection signal output by the seconddetector.

In one preferable embodiment, the optical pickup further comprises afirst collimator lens for decreasing a divergence degree of the lightemitted by the first light source; and a second collimator lens fordecreasing a divergence degree of the light emitted by the second lightsource. A first magnitude obtained by dividing a focal length of thefirst collimator lens by a focal length of the first objective lens issmaller than a second magnitude obtained by dividing a focal length ofthe second collimator lens by a focal length of the second objectivelens.

In one preferable embodiment, a focal length of the first objective lensis longer than a focal length of the second objective lens.

In one preferable embodiment, the optical pickup further comprises afirst collimator lens for decreasing a divergence degree of the lightemitted by the first light source; and a second collimator lens fordecreasing a divergence degree of the light emitted by the second lightsource. A focal length of the first collimator lens is shorter than afocal length of the second collimator lens.

An optical information apparatus according to the present inventioncomprises an optical pickup defined by any one of the above; a motor fordriving an optical disc to rotate; and an electric circuit forcontrolling the optical pickup based on a signal obtained from at leastthe first light detector of the optical pickup.

A computer according to the present invention comprises theabove-described optical information apparatus.

An optical disc player according to the present invention comprises theabove-described optical information apparatus.

A car navigation system according to the present invention comprises theabove-described optical information apparatus.

An optical disc recorder according to the present invention comprisesthe above-described optical information apparatus.

An optical disc server according to the present invention comprises theabove-described optical information apparatus.

A vehicle according to the present invention comprises theabove-described optical information apparatus.

A method according to the present invention for assembling an opticalpickup described above comprises a first step of adjusting aninclination of the entirety of the first lens holder, such that acomatic aberration on the recording face of the optical disc when alight beam from the second light source is collected by the secondobjective lens is minimized; and a second step of, in the state wherethe inclination of the entirety of the first lens holder adjusted in thefirst step is kept, adjusting an inclination of the objective lens unitwith respect to the first lens holder, such that a comatic aberration onthe recording face of the optical disc when a light beam from the firstlight source is collected by the second objective lens is minimized.

In one preferable embodiment, the optical pickup further includes athird light source for emitting light to be collected by the secondobjective lens; and the method further comprises a third step of, afterthe second step, adjusting a position in a vertical direction of thethird light source with respect to an optical axis of a light beam fromthe third light source, such that a comatic aberration on the recordingface of the optical disc when the light beam from the third light sourceis collected by the second objective lens is minimized.

An objective lens driving device according to the present inventioncomprises a movable body including a first objective lens and a secondobjective lens for converging a light beam to an optical disc, and asupport for supporting the first objective lens and the second objectivelens. An interval between an optical axis of the first objective lensand an optical axis of the second objective lens is 5 mm or less.

In one preferable embodiment, an interval between an optical axis of thefirst objective lens and an optical axis of the second objective lens is2.5 mm or greater and 5 mm or less.

An objective lens driving device according to the present inventioncomprises a movable body including a first objective lens and a secondobjective lens for converging a light beam to an optical disc, a firstlens holder for supporting the first objective lens, and a second lensholder for supporting the second objective lens. The first lens holderis secured to the second lens holder. The first lens holder has a flatside surface at a position proximate to the second objective lenssupported by the second lens holder.

In one preferable embodiment, the first lens holder has the flat sidesurface at a position facing an outer peripheral area of the opticaldisc.

In one preferable embodiment, the first objective lens is used tocollect light having a longer wavelength than light collected by thesecond objective lens.

In one preferable embodiment, the first lens holder has a cutout at aposition symmetrical to the flat side surface with respect to an opticalaxis of the first objective lens.

In one preferable embodiment, the objective lens driving device furthercomprises a projection projecting from each of the first lens holder andthe second lens holder more than the first objective lens and the secondobjective lens.

The projection is provided in an area other than an area facing an outerperipheral area of the optical disc.

Effects of the Invention

An objective lens unit according to the present invention is formed of amaterial which transmits ultraviolet. Therefore, the objective lens unitcan be fixed to a support with an ultraviolet-curable resin after theinclination of the objective lens unit is adjusted. Thus, the positionor inclination of the objective lens unit can be easily adjusted.

The objective lens unit includes an opening limiting section. Therefore,even after the inclination of the objective lens unit with respect tothe support is adjusted, the diameter of the light beam incident on theobjective lens from outside can be adjusted and kept the same by theopening limiting section.

In an optical pickup according to the present invention, the distancebetween the optical axes of the two objective lenses is 5 mm or less.Therefore, the deterioration in the servo control performance, whichwould be otherwise caused by resonance of the lens holder, can besuppressed, and a stable control is realized.

The first lens holder has a flat side surface at a position proximate tothe second objective lens. Therefore, the distance between the twoobjective lenses can be made short, and thus the distance between theoptical axes of the two objective lenses can be made short.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic side view showing an optical pickup according toEmbodiment 1 of the present invention.

FIG. 2 is a schematic isometric view showing the optical pickupaccording to Embodiment 1 of the present invention.

FIG. 3 shows a cross-section of a movable section of an objective lensdriving device usable for the optical pickup shown in FIG. 1.

FIG. 4 is an enlarged cross-sectional view of a lens unit usable for theoptical pickup shown in FIG. 1.

FIG. 5 is a partial enlarged cross-sectional view of a lens holderusable for the optical pickup shown in FIG. 1.

FIG. 6 illustrates an optical path of light incident on an openinglimiting section of the lens holder.

FIG. 7A is a cross-sectional view showing another embodiment of theopening limiting section of the lens holder.

FIG. 7B is a cross-sectional view showing still another embodiment ofthe opening limiting section of the lens holder.

FIG. 7C is a cross-sectional view showing still another embodiment ofthe opening limiting section of the lens holder.

FIG. 7D is a cross-sectional view showing still another embodiment ofthe opening limiting section of the lens holder.

FIG. 7E is a cross-sectional view showing still another embodiment ofthe opening limiting section of the lens holder.

FIG. 7F is a cross-sectional view showing still another embodiment ofthe opening limiting section of the lens holder.

FIG. 7G is a cross-sectional view showing another embodiment of the lensunit.

FIG. 8 shows an example of a focusing error signal.

FIG. 9 shows a focusing error signal obtained from an optical system fora BD in Embodiment 1.

FIG. 10 shows a focusing error signal obtained from an optical systemfor a DVD in Embodiment 1.

FIG. 11 shows a focusing error signal obtained from an optical systemfor a CD in Embodiment 1.

FIG. 12 is an isometric view showing an objective lens driving deviceaccording to Embodiment 2.

FIG. 13 is an exploded isometric view showing the objective lens drivingdevice according to Embodiment 2.

FIG. 14 is a cross-sectional view of a lens holder of the objective lensdriving device shown in FIG. 12.

FIG. 15 shows the relationship of the distance between optical axes vs.and the phase delay amount of servo control.

FIG. 16 shows a displacement frequency response characteristic in afocusing direction of the lens holder to the value of current flowing ina focusing coil in Embodiment 2.

FIG. 17 shows a displacement frequency response characteristic in afocusing direction of the lens holder to the phase delay amount of servocontrol in Embodiment 2.

FIG. 18 is a schematic view showing resonance of a conventional lensholder.

FIG. 19 shows a displacement frequency response characteristic in afocusing direction of the lens holder to the level of current flowing ina focusing coil in a conventional objective lens driving device.

FIG. 20 shows a displacement frequency response characteristic in afocusing direction of the lens holder to the phase delay amount of servocontrol in the conventional objective lens driving device.

FIG. 21 is a plan view showing a structure of an objective lens drivingdevice according to Embodiment 3.

FIG. 22 is an exploded isometric view showing the structure of theobjective lens driving device according to Embodiment 3.

FIG. 23 is a side view of a lens holder according to Embodiment 3.

FIG. 24 is a cross-sectional view taken along line A-A in FIG. 21.

FIG. 25 shows the positional relationship between the objective lensdriving device and the optical disc in Embodiment 3.

FIG. 26 shows a structure of an optical information apparatus accordingto Embodiment 4.

FIG. 27 shows a structure of a computer according to Embodiment 5.

FIG. 28 shows a structure of an optical disc player according toEmbodiment 6.

FIG. 29 shows a structure of an optical disc recorder according toEmbodiment 7.

FIG. 30 shows a structure of a server according to Embodiment 8.

FIG. 31 shows a structure of a vehicle according to Embodiment 9.

DESCRIPTION OF THE REFERENCE NUMERALS

-   -   1, 2 Lens holder    -   3 Opening limiting section    -   31, 37 a, 43 a Light source    -   34, 41 Objective lens    -   33, 39 Collimator lens    -   32, 35, 46 Optical disc    -   45 Objective lens driving device    -   56, 57, 58 Light beam

BEST MODE FOR CARRYING OUT THE INVENTION Embodiment 1

Hereinafter, an optical pickup according to Embodiment 1 of the presentinvention will be described. FIGS. 1 and 2 are respectively a schematicside view and a schematic isometric view of an optical pickup 101.

In FIG. 1, direction T is a tracking direction, and direction F is afocusing direction. Direction Y is vertical to the tracking direction.The optical pickup 101 are located such that these directionsrespectively match the tracking direction of an optical informationapparatus (optical disc apparatus), the focusing direction of theoptical information apparatus, and a direction vertical to the trackingdirection of the optical information apparatus. It is also possible tolocate the optical pickup 101 such that the direction Y and thedirection T respectively match the tracking direction of the opticalinformation apparatus and a direction vertical thereto.

The optical pickup 101 is capable of performing at least one ofrecording and reproduction on and from three optical discs 32, 35 and 46having different recording densities. The optical discs 32, 35 and 46are, for example, a DVD, a BD and a CD.

In order to perform at least one of recording and reproduction on andfrom the optical disc 35 having the highest recording density, theoptical pickup 101 includes a light source 31 and an objective lens 34.

The light source 31 emits light having a shortest wavelength (forexample, blue light). The light emitted by the light source 31 is linearpolarization of a predetermined direction. A polarization beam splitter1032 guides the light emitted by the light source 31 to a collimatorlens 33. The polarization beam splitter 1032 strongly reflects linearpolarization of the direction of the light emitted by the light source31 and transmits light of a direction perpendicular thereto at a highratio. The polarization beam splitter 1032 also provides an effect ofimproving the light utilization factor when being combined with a ¼waveplate 48 as described below.

The collimator lens 33 converts a convergence state of a light beam 56radiated by the light source 31 such that the light beam 56 becomesgenerally parallel light. A prism 60 has an inclined surface 60 a, andreflects the light beam 56 transmitted through the collimator lens 33 ina direction perpendicular to the optical disc 35. The objective lens 34converges the light beam 56 on a recording face of the optical disc 35.The light beam reflected by the recording face of the optical disc 35travels in the opposite direction to the above, is branched in adifferent direction from the direction toward the light source 31 bybranching means such as the polarization beam splitter 1032 or the like,and is incident on a light detector 36. The light detector 36 performsoptoelectric conversion on the incident light and outputs an electricsignal for obtaining an information signal or a servo signal (a focusingerror signal for focusing control or a tracking signal for trackingcontrol).

By providing the ¼ waveplate 48 between the polarization beam splitter1032 and the objective lens 34, the linear polarization reflected by thepolarization beam splitter 1032 can be converted into circularpolarization. The light beam 56, which has become circular polarizationof the opposite direction as a result of being reflected by therecording face of the optical disc 35, is converted into linearpolarization of a direction perpendicular to the original directionthereof when being incident again on the ¼ waveplate 48. Since thislight is transmitted through the polarization beam splitter 1032 at ahigh ratio, the light utilization factor can be improved.

It is also allowed to locate an axial astigmatism generation element 50such as a cylindrical lens, a toric lens or the like between thepolarization beam splitter 1032 and the light detector 36 and to locatea quadrant light detection area (not shown) in the light detector 36. Byadopting such a structure so that light provided with axial astigmatismby the axial astigmatism generation element 50 is received, a focusingerror signal generated by an axial astigmatism method can be detected.In the case where a lens power for collecting or diffusing light to theaxial astigmatism generation element 50 is added, the axial astigmatismgeneration element 50 can be moved toward an optical axis and perform anoffset adjustment of the focusing error signal. By locating adiffractive optical element (DOE) 51 between the polarization beamsplitter 1032 and the light detector 36, a stable tracking signal of aone-beam system can be obtained.

The optical pickup 101 includes a light source 37 a and an objectivelens 41. The light source 37 a is accommodated in an integral unit 37,and radiates light having a longer wavelength than the light emitted bythe light source 31 (for example, radiates red light). A collimator lens39 converts a convergence state of a light beam 57 radiated by the lightsource 37 a such that the light beam 57 becomes generally parallellight. The light beam 57 transmitted through the collimator lens 39 isreflected by an inclined surface 60 b, which is different from theinclined surface 60 a, of the prism 60 in a direction perpendicular tothe optical disc 32.

The objective lens 41 converges the light beam 57 on a recording face ofthe optical disc 32. The light beam reflected by the recording face ofthe optical disc 32 travels in the opposite direction to the above, isbranched in a different direction from the forward path by branchingmeans such as a polarization hologram 40 or the like, and is incident ona light detector (not shown) in the integral unit 37 for optoelectricconversion. Thus, an electric signal for obtaining an information signalor a servo signal (a focusing error signal for focusing control or atracking signal for tracking control) is output. By using the integralunit 37 having the light detector and the light source 37 a builttherein, the optical pickup can be reduced in size and thickness and canbe stabilized in operation.

The optical pickup 101 also includes a light source 43 a. The lightsource 43 a is accommodated in an integral unit 43, and radiates lighthaving a longest wavelength (for example, red light). The collimatorlens 39 converts a convergence state of a light beam 58 radiated by thelight source 43 a such that the light beam 58 becomes generally parallellight. The light beam 58 transmitted through the collimator lens 39 isreflected by the inclined surface 60 b of the prism 60 in a directionperpendicular to the optical disc 46.

The objective lens 41 converges the light beam 58 on a recording face ofthe optical disc 46. The light beam 58 reflected by the recording faceof the optical disc 46 travels in the opposite direction to the above,is branched in a different direction from the direction toward the lightsource 43 a by branching means such as a hologram 43 b or the like, andis incident on a light detector (not shown) in the integral unit 43 foroptoelectric conversion. Thus, an electric signal for obtaining aninformation signal or a servo signal (a focusing error signal forfocusing control or a tracking signal for tracking control) is output.By using the integral unit 43 having the light detector and the lightsource 43 a built therein, the optical pickup can be reduced in size andthickness and can be stabilized in operation.

As shown in FIGS. 1 and 2, the optical pickup 101 includes a beamsplitter 38 having a dichroic film for assembling or branching lightemitted by the light sources 37 a and 43 a. A part of the light emittedby the light source 43 a is reflected by the beam splitter 38 and isguided to a light detector 54. The rest of the light is transmittedthrough the collimator lens 39. The beam splitter 38 also transmits apart of the light reflected by the optical disc 46 and guides the partof the light to the integral unit 43. The beam splitter 38 totallyreflects the light emitted by the light source 37 a.

The prism 60 has a triangular cross-section, but the apexes (ridges whenseen from the viewpoint of the entire shape of the prism) may bechamfered to prevent chipping. With this case being encompassed, theprism 60 has a generally triangular cross-section.

In this embodiment, the optical pickup 101 is compatible to three typesof optical discs and includes three light sources. In order to realizean optical information apparatus compatible to two types of opticaldiscs, the optical pickup 101 may include either the light source 43 aor 37 a, and the light source 31.

The optical pickup 101 includes an objective lens driving device 45, andthe objective lenses 34 and 41 are located at predetermined positions inthe objective lens driving device 45. The objective lenses 34 and 41 arepreferably arranged in a line in the direction Y, in which the trackinggrooves of the optical disc extend, or in a direction perpendicular tothe tracking direction. This prevents the situation where when theobjective lens 34 or 41 accesses an outermost area or an innermost areaof the optical disc 32, 35 or 46, the objective lens 34 or 41 which isnot in use interferes a motor for rotating the optical disc 32, 35 or 46or an external area of the optical information apparatus.

The objective lens driving device (objective lens actuator) 45 can movethe objective lenses 34 and 41 either in the focusing direction Fperpendicular to the recording face of the optical discs 35, 32 and 46or in the tracking direction T of the optical discs. Thus, the objectivelens driving device 45 can adjust the light collection state of theoptical spot used for performing recording or reproduction on or fromthe optical discs 32, 35 and 46. The objective lens driving device 45can also move the optical spot in the tracking direction.

FIG. 3 shows a cross-section of a movable section of the objective lensdriving device 45. The movable section supports and secures theobjective lenses 34 and 41. A driving mechanism such as a coil, a magnetor the like (not shown) moves the movable section in the focusingdirection or the tracking direction, and thus the objective lenses 34and are moved in the focusing direction or the tracking direction.

After the objective lenses 34 and 41 are both secured in the movablesection, the orientation of the objective lens 34 or 41 cannot beadjusted independently. Therefore, the direction of the optical axis ofthe objective lens 41 with respect to the optical axis of the objectivelens 34 needs to be adjusted beforehand.

In this embodiment, the movable section includes an objective lens unit10 in which the objective lens 34, a lens holder 1, and the objectivelens 41 are supported. The orientation of the objective lens unit 10 canbe adjusted with respect to the first lens holder 1 to which theobjective lens 34 is secured. For this end, the objective lens unit 10includes a lens holder 2. The lens holder 2 supports and secures theobjective lens 41. The lens holder 2 has a curved bottom part 2 r, andis supported and secured such that a curved recess 1 r of the lensholder 1 contacts the bottom part 2 r of the lens holder 2.

Since the bottom part 2 r of the lens holder 2 is curved, the lensholder 2 and the lens holder 1 can be secured after the inclination ofthe lens holder 2 is adjusted inside the recess 1 r of the lens holder1.

The inclination of the lens holder 2 is adjusted by, for example, thefollowing procedure. First, the inclination of the lens holder 1 isadjusted such that when the light beam 56 reflected by the inclinedsurface 60 a in FIG. 1 is collected by the objective lens 34, thecomatic aberration is minimized or the convergence spot is mostsymmetric with respect to the optical axis. This adjustment is performedby adjusting the position of the lens holder 1 in the movable section orby adjusting the inclination of the objective lens driving device 45with respect to the optical axis of the light beam 56 reflected by theinclined surface 60 a.

Then, in this state, the inclination of the lens holder 2 having theobjective lens 41 secured thereto is adjusted with respect to the lensholder 1. This adjustment is performed such that when the light beam 57reflected by the inclined surface 60 b is collected by the objectivelens 41, the comatic aberration is minimized or the convergence spot ismost symmetric with respect to the optical axis. Then, the position ofthe light source 43 a is adjusted in a direction vertical to the opticalaxis, such that when the light beam 58 reflected by the inclined surface60 b is collected by the objective lens 41, the comatic aberration isminimized or the convergence spot is most symmetric with respect to theoptical axis.

According to the conventional art disclosed in Patent Document 1, theinclination of each of the two objective lenses is determined such thatthe optical axes of the two objective lenses are parallel to each other.With this method, however, the comatic aberration generated by aproduction error of the objective lenses or the comatic aberration ofthe light beam generated by a production error of the other opticalcomponents is left remaining, and so the convergence beam is distorted.

By contrast, with the adjusting method according to this embodiment, thecomatic aberration can be comprehensively reduced or the shape of theconvergence beam can be optimized. Therefore, an optical pickupexhibiting superb recording or reproduction performance on or from aplurality of different types of optical discs is realized.

After the inclination of the lens holder 2 with respect to the lensholder 1 is adjusted as described above, the lens holder 2 is bonded tothe lens holder 1 with an adhesive 22 as shown in FIG. 3. For bondingelectronic components, thermosetting adhesives are widely used ingeneral. However, use of a thermosetting adhesive requires a hightemperature tank, which requires extra production equipment. Where anadhesive curable over-time is used, the time necessary for curing ispreferably as short as possible from the viewpoint of shortening theproduction time. However, where the time necessary for curing is short,the adhesive is progressively cured while the inclination of the lensholder 2 is adjusted. This makes the adjustment difficult.

In this embodiment, it is preferable to use an ultraviolet-curable resinas the adhesive 22 in consideration of these points. The adhesive 22 ispreferably applied also to a gap between the lens holder 1 and the lensholder 2 to strongly bond the lens holder 1 and the lens holder 2 toeach other. The lens holder 2 is preferably formed of a material whichtransmits ultraviolet in order to cure the adhesive 22 applied to thegap between the lens holder 1 and the lens holder 2. A polyolefin-basedresin or a polycarbonate-based resin transmits ultraviolet and hasstable physical properties while costing low, and so is suitable as amaterial for forming the lens holder 2. The lens holder 1 is formed of amaterial which does not transmit visible light or ultraviolet.

After the optical axis of the objective lens 41 is adjusted by theabove-described procedure, the adhesive 22 is irradiated withultraviolet to be cured and thus the lens holder 2 is fixed to the lensholder 1. This prevents the situation where the adjustment is madedifficult as a result of the adhesive 22 being cured while the opticalaxis of the objective lens 41 is adjusted. Since an ultraviolet-curableresin can be cured in a relatively short time, the time required for theadjustment is shortened. Since the lens holder 2 transmits ultraviolet,the adhesive 22 can be cured uniformly. Since only a light source forproviding the ultraviolet is additionally needed, the cost for theequipment is low. There is also an effect that the time necessary forbonding and fixing the lens holders is short.

The objective lens fixed to the lens holder 2 is preferably included inan optical system used for recording or reproduction on or from anoptical disc having a relatively low recording density. The controlprecision required for performing recording or reproduction on or froman optical disc having a low recording density may be lower than thecontrol precision required for performing recording or reproduction onor from an optical disc having a high recording density. Therefore, theprecision required for adjusting the optical axis of the objective lensmay also be lower for an optical disc having a low recording densitythan for an optical disc having a high recording density. For thisreason, the optical axis is adjusted more easily where the objectivelens fixed to the lens holder 2 is included in an optical system usedfor an optical disc having a relatively low recording density.

As shown in FIG. 3, the lens holder 1 includes an opening limitingsection 14 projecting inward in a through-hole which defines the opticalpath of the light beam incident on the objective lens 34. The openinglimiting section 14 is provided to define an opening 1 h in order tocause a desired size of light beam to be incident on the objective lens34. The opening limiting section 14 is also formed of a material whichdoes not transmit visible light or ultraviolet.

In the case where the lens holder 2 is formed of a material whichtransmits ultraviolet, the lens holder 2 also transmits visible light.This makes it difficult to cause a desired size of light beam to beincident on the objective lens 41 and thus to obtain a desired numericalaperture (NA). According to the present invention, the objective lensunit includes an opening limiting section 3. Now, with reference to FIG.4, the opening limiting section 3 will be described in detail.

FIG. 4 is an enlarged cross-sectional view of the lens unit 10. As shownin FIG. 4, the lens holder 2 has a through-hole having a first opening 2b and a second opening 2 a and defined by an inner surface 2 f. Theobjective lens 41 is supported by the lens holder 2 so as to block thefirst opening 2 b, and light transmitted through the through-hole fromthe second opening 2 a is incident on the objective lens 41. The openinglimiting section 3 is provided along a circumferential direction of theinner surface 2 f of the through-hole and has a shape projecting towarda central axis 2 c of the through-hole. Owing to this, the openinglimiting section 3 defines a diameter 2 h of the light beam incident onthe objective lens 41. The central axis 2 c of the through-holegenerally matches a center 41 c of the objective lens 41. The opening 2h defined by the opening limiting section 3 is set to have a size equalto or smaller than effective diameter D within which the objective lens41 acts as a lens.

As described above, the lens holder 2 transmits ultraviolet and visiblelight. Therefore, where the opening limiting section 3 is formed of thesame material as the lens holder 2 integrally therewith, the incidentlight is also transmitted through the opening limiting section 3. As aresult, the light cannot be shielded from being incident on theobjective lens 41 and a predetermined light diameter cannot be obtained.In order to avoid this, the lens holder has a structure for guiding thelight incident on the opening limiting section 3 from the second opening2 a of the opening limiting section 3 in a direction away from theoptical axis of the objective lens 41.

FIG. 5 is a partial enlarged cross-sectional view of the lens holder 2.As shown in FIG. 5, the opening limiting section 3 has a trapezoidalcross-section. Owing to two inclined sides of the trapezoid, the openinglimiting section 3 includes a first ring-shaped inclined surface 3 a anda second ring-shaped inclined surface 3 b both projecting in thethrough-hole of the lens holder 2. The first ring-shaped inclinedsurface 3 a and the second ring-shaped inclined surface 3 b are inclinedwith respect to the inner surface 2 f so as to face the first opening 2b and the second opening 2 a respectively. A side surface 3 c of theopening limiting section 3 is generally parallel to the inner surface 2f.

As shown in FIG. 5, among the light incident on the first opening 2 b,the light 57 incident on the opening limiting section 3 is transmittedthrough the opening limiting section 3 from the first ring-shapedinclined surface 3 a. At this point, the light 57 is refracted in adirection away from the optical axis 41 c by the first ring-shapedinclined surface 3 a. Thus, the light incident on the opening limitingsection 3 can be prevented from being incident on the objective lens 41.As a result, as shown in FIG. 4, only the light having the diameter 2 hdefined by the opening limiting section 3 can be incident on theobjective lens 41.

As represented with the dashed line in FIG. 5, the first ring-shapedinclined surface 3 a and the second ring-shaped inclined surface 3 b ofthe opening limiting section 3 may be a part of two spherical surfacesof a concave lens which generally matches the optical axial 41 c. Evenwith such a shape of the first ring-shaped inclined surface 3 a and thesecond ring-shaped inclined surface 3 b, the light 57 incident on theopening limiting section 3 can be refracted in a direction away from theoptical axis 41 c. With such a structure, even where the openinglimiting section 3 is formed of a material which transmits visible lightand ultraviolet, the light can be shielded by the opening limitingsection 3 to cause light having a desired diameter to be incident on theobjective lens 41.

The opening limiting section 3 is provided in the through-hole, throughwhich the light incident on the objective lens 41 supported by the lensholder 2 passes. Therefore, the opening limiting section 3 can belocated proximate to the objective lens 41, and the shift of the centerof the opening limiting section 3 from the optical axis of the objectivelens 41 is decreased. In addition, when adjusting the inclination of theobjective lens 41 with respect to the lens holder 1, the adjustment isperformed for the entire lens unit including the objective lens 41 andthe lens holder 2. Therefore, the shift of the center of the openinglimiting section 3 from the optical axis of the objective lens 41 doesnot occur, and the diameter of the light incident on the objective lens41 is always determined by the opening limiting section 3. This canminimize the error in the diameter of the light incident on theobjective lens 41.

In order to improve the effect of shielding the light provided by theopening limiting section 3, the total reflection of light on the secondring-shaped inclined surface 3 b may be used. Specifically, this is doneas follows. As shown in FIG. 6, an angle made by the light 57 incidenton the first ring-shaped inclined surface 3 a and the normal to thefirst ring-shaped inclined surface 3 a is set to A1, and an angle madeby the light traveling inward into the opening limiting section 3 fromthe first ring-shaped inclined surface 3 a and the normal to the firstring-shaped inclined surface 3 a is set to A2. An angle made by thelight incident on the second ring-shaped inclined surface 3 b and thenormal to the second ring-shaped inclined surface 3 b is set to A4, andan angle made by the light output from the second ring-shaped inclinedsurface 3 b and the normal to the second ring-shaped inclined surface 3b is set to A3. A refractive index of the material forming the openinglimiting section 3 is set to n.

By the Snell's law, these angles fulfill expressions (1) and (2).sin(A1)=n·sin(A2)  (1)A4=A3+(A1−A2)  (2)

The condition under which total reflection occurs on the secondring-shaped inclined surface 3 b is represented by expression (3).n·sin(A4)>1  (3)

By substituting expression (2) for expression (3), the relationship ofexpression (4) is obtained.n·sin(A3+(A1−A2))>1  (4)

Namely, as long as angle A1 and angle A3 fulfill the relationships ofexpressions (1) and (4), all the light incident on the first ring-shapedinclined surface 3 a can be refracted in a direction away from theoptical axis 41 c and can be prevented from being incident on theobjective lens 41.

Such a structure prevents unnecessary light beams from being incident onthe objective lens 41, and thus provides an effect of completelyremoving stray light components.

The opening limiting section 3 provided in the lens holder 2 may have astructure other than that shown in FIG. 5 or 6.

As shown in FIG. 7A, an opening limiting section 63 provided in a lensholder 62 includes two ring-shaped inclined surfaces 63 d and 63 e,concentrically provided, a second ring-shaped inclined surface 63 b, anda side surface 63 c. The ring-shaped inclined surfaces 63 d and 63 e areinclined to face the first opening 2 b, and are discontinuous to eachother. The ring-shaped inclined surfaces 63 d and 63 e act as a firstring-shaped inclined surface 63 a, and guide the light incident on theopening limiting section 63 in a direction away from the optical axis 41c as described with reference to FIG. 5.

By forming the first ring-shaped inclined surface 63 a of the twodiscontinuous ring-shaped inclined surfaces 63 d and 63 e, the incliningangle of each of the ring-shaped inclined surfaces 63 d and 63 e withrespect to an inner surface 63 f can be decreased. Therefore, the lightincident on the opening limiting section 63 can be refracted in adirection further away from the optical axis 41 c. In addition, lengthL1 necessary to form the ring-shaped inclined surfaces 63 d and 63 e onthe inner surface 63 f can be shorter than the length necessary forforming one ring-shaped surface inclined at the same angle. Accordingly,the length of the lens holder 62 in the direction of the through-holecan be decreased. This is suitable to realize a thin optical informationapparatus.

As shown in FIG. 7B, an opening limiting section 65 provided in a lensholder 64 includes a ring-shaped diffraction grating 65 a provided toface the first opening 2 b, a second ring-shaped inclined surface 65 b,and a side surface 65 c. The ring-shaped diffraction grating 65 a canrefract the light incident on the opening limiting section 65 in adirection away from the optical axis 41 c. The diffraction grating 65 acan set the diffraction direction of light by a convex and concavepattern thereof. Length L2 necessary to provide the diffraction grating65 a on an inner surface 65 f can be shorter than the length necessaryfor forming the first ring-shaped inclined surface. Accordingly, thelength of the lens holder 64 in the direction of the through-hole can bedecreased. This is suitable to realize a thin optical informationapparatus.

The opening limiting section 3 shown in FIG. 5 has the first ring-shapedinclined surface 3 a and the second ring-shaped inclined surface 3 b,but the opening limiting section 3 may include either one of thering-shaped inclined surfaces. As shown in FIG. 7C, an opening limitingsection 67 provided in a lens holder 66 includes a vertical surface 67a, a second ring-shaped inclined surface 67 b, and a side surface 67 c.The vertical surface 67 a is vertical to an inner surface 67 f, and doesnot refract the light incident thereon. However, by decreasing theinclining angle of the second ring-shaped inclined surface 67 b withrespect to the inner surface 67 f, the light 57 can be refracted by thesecond ring-shaped inclined surface 67 b to be away from the opticalaxis 41 c.

As shown in FIG. 7C, an angle made by the direction in which the light57 is incident on the second ring-shaped inclined surface 67 b and thenormal to the second ring-shaped inclined surface 67 b is set to A3. Aslong as angle A3 fulfills expression (5), the light 57 is totallyreflected by the second ring-shaped inclined surface 67 b and thus canbe prevented from being incident on the objective lens 41.n·sin(A3)>1  (5)

As shown in FIG. 7D, an opening limiting section 69 provided in a lensholder 68 includes a first ring-shaped inclined surface 69 a, a verticalsurface 69 b, and a side surface 69 c. The opening limiting section 69can refract the light 57 by the first ring-shaped inclined surface 69 ain a direction away from the optical axis 41 c.

The cross-section of the opening limiting section may be curved. Asshown in FIG. 7E, an opening limiting section 72 provided in a lensholder 71 has a cross-section, taken along the central axis of thethrough-hole, which has a shape of a part of an ellipse projectingtoward the central axis. The opening limiting section 72 having such across-section also can refract the light 57 in a direction away from theoptical axis 41 c. The lens holder 71 having such a shape can be easilymolded by a method such as injection molding or the like, and thus canreduce the production cost of the lens holder 71.

The opening limiting sections described above have a structure which canbe formed integrally with the lens holder 71, and can be produced at lowcost. For shielding the light more certainly, a light shielding surfacemay be provided in the opening limiting section. As shown in FIG. 7F, anopening limiting section 74 provided in a lens holder 73 includes aring-shaped projection including generally vertical surfaces 74 a and 74b and a side surface 74 c. The ring-shaped projection projects from aninner surface 74 f toward the optical axis 41 c. On the surface 74 afacing the first opening 2 b, a ring-shaped shielding surface 75 isprovided. The shielding surface 75 may be one surface of a ring-shapedsheet formed of a resin containing a black coating material or graphite,or a colored resin, for example. Alternatively, the entire openinglimiting section may be formed of a resin containing a black coatingmaterial or graphite, or a colored resin. Instead of the shieldingsurface 75 for shielding the light, a scattering surface for scatteringthe light may be provided.

Such a surface is formed of a different material from that of the lensholder 2 and so the production cost is slightly raised, but can shieldunnecessary light with a higher level of certainty because the effect ofshielding the light is high.

In this embodiment, the lens unit includes a lens holder and anobjective lens. The lens unit may include other optical elements.

For example, as shown in FIG. 7G, a lens unit 80 includes the objectivelens 41, a lens holder 83, an opening limiting section 82 provided inthe lens holder, and an optical element 84.

The opening limiting section 82 may have any of the various structuresdescribed above. As the optical element 84, any of various opticalelements such as a diffractive lens is usable. Since the lens holder 83transmits ultraviolet, the lens holder 83 and the optical element 84 canbe bonded together with an ultraviolet-curable resin. After theobjective lens 41, the opening limiting section 82 and the opticalelement 84 are appropriately adjusted in terms of inclination andposition, ultraviolet is radiated thereto to fix the optical element 84to the lens holder 83. Thus, an optical unit having an adjusted opticalaxis can be obtained.

Now, with reference to FIGS. 1 and 2, designing of the optical systemaccording to this embodiment will be described. The optical pickup 101is compatible to three optical discs 32, 35 and 46 having differentrecording densities. The optical discs 32, 35 and 46 are, for example, aDVD, a BD and a CD.

For performing a recording or reproduction operation on or from aplurality of optical discs having different recording densities, therequired control precision varies. In general, for performing arecording or reproduction operation on or from an optical disc having alow recording density, a high level of control precision is notrequired. However, due to the low recording density, the objective lensis required to move in a large area. By contrast, for performing arecording or reproduction operation on or from an optical disc having ahigh recording density, a high control precision is required, but thearea in which the objective lens moves may be small. For example,between a BD having a high recording density and a CD having a lowrecording density, the recording density is different by about 40 timesand therefore the required control precision and the required movingarea of the objective lens are also significantly different.

For the above-described reasons, when a disc having a low recordingdensity is mounted on an optical information apparatus which tolerates arelatively large shape distortion and is rotated by a spindle motor, theposition of the recording face is changed up and down due to thedistortion of the shape of the disc. Namely, a “face shake” occurs. Inorder to start focusing control with certainty for a disc with a largeface shake, the dynamic range of a focusing error signal needs to belarge. Specifically, as shown in FIG. 8, it is desirable that theinterval between defocus point A at which the focusing error signal hasthe maximum strength, and focus point B at which the focusing errorsignal has the minimum strength, is large.

By contrast, with an optical disc having a high recording density, theoptical spot formed on the recording face thereof is small and the focaldepth thereof is shallow. Hence, focusing control needs to be performedmore precisely. The interval between defocus point A and the focus pointB in FIG. 8 needs to be small and the detection sensitivity betweenpoints A and B needs to be high.

In this embodiment, in order to address these issues, an optical systemfor each optical disc is designed as follows.

Specifically, optical systems for transmitting the light beams 56, 57and 58 are designed such that the defocusing detection range of afocusing error signal, obtained by directing the light beam 57 or thelight beam 58 having a longer wavelength than the light beam 56 towardthe optical disc and detecting the reflected light by a light detector,is larger than the defocusing detection range of a focusing errorsignal, obtained by directing the light beam 56 toward the optical discand detecting the reflected light by a light detector.

Preferably, focal length f1 of the objective lens is set to 1 mm to 1.8mm, and focal length fC1 of the collimator lens 33 is set to 14 mm to 30mm. Focal length f2 of the objective lens 41 is set to 2 mm to 3 mm, andfocal length fC2 of the collimator lens 39 is set to 10 mm to 20 mm.

Furthermore, within these ranges, a first magnification obtained bydividing focal length fC1 of the collimator lens 33 by focal length f1of the objective lens 34 is set to be larger than a second magnificationobtained by dividing focal length fC2 of the collimator lens 39 by focallength of the objective lens 41. Namely, the focal lengths are set tofulfill the relationship of expression (6).fC1/f1>fC2/f2  (6)

In order to fulfill the relationship of expression (6), it is preferablethat focal length f2 of the objective lens 41 is made longer than focallength f1 of the objective lens 34, or that focal length Cf2 of thecollimator lens 39 is made shorter than focal length fC1 of thecollimator lens 33.

By selecting the focal lengths of the objective lenses 34 and 41 and thecollimator lenses 33 and 39 in this manner, the following is madepossible. For performing recording or reproduction on or from theoptical disc 35 having a high recording density, highly precise focusingcontrol is realized. For performing recording or reproduction on or fromthe optical disc 32 or 46 having a low recording density, the defocusingdetection sensitivity can be reduced and the defocusing detection areacan be enlarged. Therefore, even when the face shake of the rotatingoptical disc is large, focusing control can be started with certainty.

Especially, a relay lens 44 may be given a convex lens function. In thiscase, the angle at which the light source 43 a is grasped through theopening of the objective lens 41, i.e., the light source numericalaperture (NA) may be converted from a large value obtained in thevicinity of the light source into a small value obtained in the vicinityof the collimator lens 39. In this way, the magnitude obtained with theinfrared light having the longest wavelength in this embodiment isminimized. As a result, the effect of reducing the defocusing detectionsensitivity and maximizing the defocusing detection range is provided.

A focusing error signal obtained by an optical pickup including theoptical systems designed in this manner will be schematically describedbelow. FIG. 9 shows a focusing error signal obtained by directing bluelight from the light source 31 toward a BD. FIG. 10 shows a focusingerror signal obtained by directing red light from the light source 37 atoward a DVD. FIG. 11 shows a focusing error signal obtained bydirecting infrared light from the light source 43 a toward a CD.

In FIGS. 9 through 11, the horizontal axis represents the defocusamount, i.e., the distance between the recording face and theconvergence spot in the direction of the optical axis (focusingdirection), and the vertical axis represents the strength of thefocusing error signal.

As shown in FIG. 9, regarding the focusing error signal for the BD, thedynamic range, namely, the interval between the defocus amounts at whichthe strength of the focusing error signal is maximized and minimized isset to about 2 μm. As shown in FIG. 10, regarding the focusing errorsignal for the DVD, the dynamic range, namely, the interval between thedefocus amounts at which the strength of the focusing error signal ismaximized and minimized is set to about 4 μm.

Such settings are realized by setting the relationship of the focallengths of the objective lens and the collimator lens, i.e., themagnification. With such settings, a highly sensitive focusing errorsignal for the BD is obtained and highly precise focusing control can beperformed.

At this point, as shown in FIG. 11, regarding the focusing error signalfor the CD, the dynamic range, namely, the interval between the defocusamounts at which the strength of the focusing error signal is maximizedand minimized, i.e., the defocusing detection range is about 6 μm. Thisis an effect obtained by reducing the magnification from that of the DVDby the relay lens. In this way, even for a CD having a large face shake,focusing control can be started with stably.

Alternatively, the optical system may be designed such that the focallength of the objective lens 41 for infrared light is longer than thefocal length of the objective lens 41 for red light. In this case also,the effect of reducing the defocusing detection sensitivity andenlarging the defocusing detection range can be obtained.

Embodiment 2

In an optical pickup including a plurality of objective lenses, aplurality of through-holes through which an optical beam passes needs tobe formed in the lens holder. Especially for performing recording orreproduction on or from an optical disc having a high recording density,an objective lens having a larger numerical aperture needs to be used.Accordingly, the corresponding through-hole needs to be larger. Due tothe large through-hole, such a lens holder has a decreased rigidity andthus is likely to be resonated at a predetermined frequency. The opticalpickup according to this embodiment includes an objective lens drivingdevice having a structure for suppressing the deterioration in servoperformance, which would be otherwise caused by resonance of the lensholder.

FIGS. 12 and 13 are respectively an isometric view and an explodedisometric view of an objective lens driving device of an optical pickupaccording to this embodiment. Like in Embodiment 1, arrows F, T and Yrespectively represent a focusing direction, a tracking direction and atangential direction of an optical disc (not shown). Arrow R representsa tilt direction, which is a rotation direction around the Y axis. Thefocusing direction F, the tracking direction T, and the direction Ycross one another perpendicularly, and respectively correspond to thedirections of coordinate axes of a three-dimensional Cartesiancoordinate system.

An objective lens driving device 102 according to this embodimentincludes a movable body 251. The movable body 251 includes an objectivelens 201, an objective lens 202, a lens holder 203, a first print coil204, a second print coil 205, and terminal plates 208.

The lens holder 203 is formed of a resin or the like, and supports theobjective lenses 201 and 202. The objective lens 201 is used to performrecording or reproduction on or from an optical disc having a lowrecording density such as a CD, a DVD or the like. The objective lens202 is used to perform recording or reproduction on or from an opticaldisc having a high recording density such as a BD or the like. On twoside surfaces of the lens holder 203 parallel to the direction T, thefirst print coil 204 and the second print coil 205 are attached. On twoside surfaces of the lens holder 203 parallel to the tracking directionY, the terminal plates 208 are attached.

The first print coil 204 and the second print coil 205 each have a coilstructure provided by attaching a conductive member to a respectivesubstrate in a spiral manner around an axis parallel to the direction Y.

In the first print coil 204, a first focusing coil section 204 a and afirst tracking coil section 204 b are arranged in the tracking directionT. In the second print coil 205, a second focusing coil section 205 aand a second tracking coil section 205 b are arranged in the trackingdirection T.

The first focusing coil section 204 a and the second focusing coilsection 205 a are located at positions shifted in opposite directions toeach other by an equal distance with respect to a plane which includesthe direction Y and is vertical to the tracking direction T.Furthermore, the first focusing coil section 204 a and the secondfocusing coil section 205 a are distanced from each other in thedirection Y. The first tracking coil section 204 b and the secondtracking coil section 205 b are located in substantially the samerelationship. The first print coil 204 and the second print coil 205 canfulfill the above relationship by including the same components and bybeing located at positions rotationally symmetric with respect to thedirection Y.

Both of two terminals of the first focusing coil section 204 a and bothof two terminals of the second focusing coil section 205 a areindependently connected to a control circuit (not shown) via theterminal plates 208 and wires 209. The first tracking coil section 204 band the second tracking coil section 205 b are connected to each otherin series and connected to the control circuit via the terminal plates208 and the wires 209.

The optical pickup 102 further includes a first magnet 206 and a secondmagnet 207 for driving the movable body 251. The first magnet 206 andthe second magnet 207 each include four areas, divided by two lines inthe focusing direction F and the tracking direction T used as borders.Each two areas adjacent to each other along each border are magnetizedwith different polarities.

The first magnet 206 is located to face the first print coil 204 at aposition at which a central line 204 c of the focusing coil section 204a of the first print coil 204 and a central line 204 d of the trackingcoil section 204 b of the first print coil 204 match the bordersdividing the four areas, and is secured to a yoke 210. Similarly, thesecond magnet 207 is located to face the second print coil 205 at aposition at which a central line 205 c of the focusing coil section 205a of the second print coil 205 and a central line 205 d of the trackingcoil section 205 b of the second print coil 205 match the bordersdividing the four areas, and is secured to another yoke 210.

The first magnet 206 and the second magnet 207 are preferably the samein all of the material, shape, magnetization pattern and magnetizationstrength, and the magnetic fields generated by the first magnet 206 andthe second magnet 207 are generally the same.

The two terminals of the first focusing coil section 204 a, the twoterminals of the second focusing coil section 205 a, and two terminal ofthe assembly of the tracking coil 204 b and the tracking coil 205 bconnected in series, i.e., six terminals in total, are connected to tipsof the six wires 209 via the terminal plates 208. Bases of the wires 209are secured to a substrate 213 via a suspension holder 212. The yoke210, the suspension holder 212 and the substrate 213 are secured to abase 211. The wires 209 are formed of an elastic metal material such asberyllium copper, bronze or the like, and wires or rods having, forexample, a circular, polygonal or elliptical cross-section are used asthe wires 209. The support center of the wires 209 is set to generallymatch the center of gravity of the movable body.

The objective lenses 201 and 202 are arranged on the lens holder 203 inthe direction Y. The objective lens 201 is located on the base side ofthe wires 209 with respect to the support center of the wires 209, andthe objective lens 202 is located on the tip side of the wires 209 withrespect to the support center of the wires 209.

The first print coil 204 and the first magnet 206 are located on thebase side of the wires 209, and the second print coil 205 and the secondmagnet 207 are located on the tip side of the wires 209. Namely, thefirst print coil 204 and the first magnet 206 are located on the side ofthe objective lens 201 in the direction Y, and the second print coil 205and the second magnet 207 are located on the side of the objective lens202 in the direction Y.

FIG. 14 shows a cross-section of the lens holder 203 taken along a planeparallel to the direction Y. The objective lenses 201 and 202respectively have optical axes K1 and K2. In the lens holder 203,through-holes are located at positions corresponding to the objectivelenses 201 and 202, along the optical axes K1 and K2. The through-holesare each provided for obtaining an optical path of a light beam to betransmitted through the respective objective lens.

The present inventor conducted various examinations on the rigidity andresonance of the lens holder. As a result, it was found that as thethrough-hole formed in the lens holder is enlarged, the rigidity of thelens holder may be reduced, but an important cause of deterioration inservo performance is the resonant frequency. It was found that theresonant frequency varies depending on distance p between the opticalaxes K1 and K2, and the deterioration in servo performance can besuppressed by setting distance p at an appropriate value.

FIG. 15 shows the relationship of distance p between the optical axes K1and K2 vs. the phase delay amount caused to servo control by theresonance. As shown in FIG. 15, as the distance is shorter, the delayamount is smaller. It may be considered that when distance p isshortened, the rigidity of the lens holder is decreased and so theadverse affect by the resonance is increased. However, the phase delayis not caused by the rigidity decrease itself but depends on theresonant frequency.

As long as the phase delay amount is 10 degrees or less, the servocontrol characteristic is not significantly deteriorated and the controldoes not become unstable. Accordingly, as shown in FIG. 15, it ispreferable that distance p between the optical axes K1 and K2 is set to5 mm or less. Where distance p is set to 5 mm or less, generation of aphase delay in the servo control, which would be otherwise caused byresonance, is suppressed, and thus the phase delay amount can bedecreased.

The effect of suppressing the adverse affect caused by resonance isprovided as long as distance p is 5 mm or less. A preferable lower limitof distance p depends on the diameters of the objective lenses 201 and202.

The light beam is emitted from an optical block (not shown) locatedbelow the base 211 shown in FIG. 13. The shift of the optical axis ofthe light beam from the optical axis of the objective lens generallyneeds to be 10% or less of the diameter in order to suppress thedeterioration in the recording or reproduction signal. A preferabledistance by which the objective lens moves in the tracking direction tofollow the track of the optical disc is 0.2 mm. From these points, theeffective diameter of the objective lens is preferably about 2 mm.Around the objective lens, a flat section having a width of at least 0.2mm for positioning needs to be provided. An interval of at least 0.1 mmis necessary between the two objective lenses for the designing reasons.

Based on these factors, distance p is preferably 2.5 mm or greater.Namely, distance p is preferably 2.5 mm or greater and 5 mm or less.More preferably, distance p is 3.4 mm or greater and 3.8 mm or less.Where distance p is set in this range, the phase shift amount of servocontrol by resonance is about 5 degrees and a characteristic equivalentto that without resonance can be obtained.

The objective lens driving device operates as follows. When an electriccurrent flows to the focusing coil section 204 a of the first print coil204 and the focusing coil section 205 a of the first print coil 205, anelectromagnetic force is generated between the first magnet 206 and thesecond magnet 207. Then, the objective lens is driven in the focusingdirection F.

FIG. 16 shows the displacement frequency response characteristic in thefocusing direction F of the lens holder 203 to the value of currentflowing to the focusing coil 204 a. FIG. 17 shows the displacementfrequency response characteristic in the focusing direction F of thelens holder 203 to the phase delay amount of servo control. As shown inFIG. 16, as the displacement frequency is increased, the current valuedecreases, but almost no disturbance is occurred by the frequency. Asshown in FIG. 17, the phase delay amount is 5 degrees or less over arange of about several hundred hertz to 10000 Hz, which means almost nophase delay occurs. This indicates that the phase delay does not occurin the focusing control and a superb control is realized. This alsoindicates that the control does not become unstable due to the phasedelay and a stable control is realized.

FIG. 18 schematically shows that resonance is generated in aconventional lens holder 114 including objective lenses 116 and 117. Inthe conventional objective lens driving device, the distance between thetwo objective lenses is about 6 mm. FIGS. 19 and 20 respectively showthe displacement frequency response characteristic in the focusingdirection F of the lens holder to the current value, and thedisplacement frequency response characteristic in the focusing directionF of the lens holder to the phase delay amount of servo control, both inthe conventional objective lens driving device. As shown in FIG. 19, thecurrent value is rapidly decreased at several tens of thousand hertz.This is an influence of the resonance of the lens holder. As shown inFIG. 20, the phase delay amount is increased due to the resonance in thevicinity of several tens of thousand hertz. Due to the influencethereof, the phase delay amount is increased also in the vicinity of10000 Hz.

As can be understood, according to this embodiment, the resonantfrequency of the lens holder can be appropriately controlled and alsothe phase delay amount of servo control can be decreased by decreasingthe distance between two objective lenses. The objective lens drivingdevice according to this embodiment is capable of realizing a highlyprecise and stable servo control.

The objective lens driving device according to this embodiment ispreferably usable in the optical pickup in Embodiment 1 as the objectivelens driving device 45. This realizes an optical pickup providing theeffects of this embodiment in addition to the effects of Embodiment 1.

Embodiment 3

In this embodiment, an objective lens driving device having a structurepreferable for realizing the distance between two objective lensesdescribed in Embodiment 2 will be described.

FIGS. 21 and 22 are respectively a plan view and an exploded isometricview showing a structure of an objective lens driving device 103according to this embodiment. In FIGS. 21 and 22, directions T, Y and Frepresent the focusing direction, the tracking direction, and thetangential direction of an optical disc like in Embodiment 2.

The objective lens driving device 103 includes a movable body includingan objective lens 301, an objective lens 302, a lens holder 303,focusing coils 304 a through 304 d, and tracking coils 305 a and 305 b.

The lens holder 303 is formed of a resin, and supports the objectivelenses 301 and 302. The objective lens 301 is used to perform recordingor reproduction on or from an optical disc having a low recordingdensity such as a CD, a DVD or the like. The objective lens 302 is usedto perform recording or reproduction on or from an optical disc having ahigh recording density such as a BD or the like.

On two side surfaces of the lens holder 303 parallel to the trackingdirection T, the focusing coils 304 a through 304 d and the trackingcoils 305 a and 305 b are attached. On two side surfaces of the lensholder 303 parallel to the direction Y, the terminal plates 308 areattached. The focusing coils 304 a and 304 c are connected to each otherin series, and both of two terminals of this assembly are connected to acontrol circuit (not shown) via the terminal plates 308, wires 309 and asubstrate 313. Similarly, the focusing coils 304 b and 304 d areconnected to each other in series, and both of two terminals of thisassembly are connected to the control circuit (not shown) via theterminal plates 308, the wires 309 and the substrate 313.

The tracking coils 305 a and 305 b are connected to each other inseries, and both of two terminals of this assembly are connected to thecontrol circuit (not shown) via the terminal plates 308, the wires 309and the substrate 313.

The objective lens driving device further includes a first magnet 306and a second magnet 307 for driving the movable body. The first magnet306 and the second magnet 307 each include a plurality of areas dividedby borders so as to correspond to the focusing coils 304 a through 304 dand the tracking coils 305 a and 305 b. Each two areas adjacent to eachother along each border are magnetized with different polarities. Thefirst magnet 306 and the second magnet 307 are secured to yokes 310.

Bases of the wires 309 are secured to the substrate 313 via a suspensionholder 312. The yokes 310, the suspension holder 312 and the substrate313 are secured to a base 311. The wires 309 are formed of an elasticmetal material such as beryllium copper, bronze or the like, and wiresor rods having, for example, a circular, polygonal or ellipticalcross-section are used as the wires 309. The support center of the wires309 is set to generally match the center of gravity of the movable body.

The objective lenses 301 and 302 are arranged on the lens holder 303 inthe direction Y. The objective lens 301 is located on the base side ofthe wires 309 with respect to the support center of the wires 309, andthe objective lens 302 is located on the tip side of the wires 309 withrespect to the support center of the wires 309.

FIG. 23 is a side view of a lens holder 314, and FIG. 24 is across-sectional view of the lens holder 303 taken along line A-A in FIG.21. As shown in FIGS. 22 and 24, the lens holder 303 has a sphericalrecess 303 a, on which the objective lens 301 is to be mounted. Theobjective lens 301 is located on the spherical recess 303 a of the lensunit 303 in the form of a lens unit mounted on the lens holder 314having a spherical projection 314 a. Hence, as described in Embodiment1, the angle of the objective lens 301 can be adjusted independentlyfrom the objective lens 302 by sliding the spherical projection 314 a ofthe tilting holder 314 on, and in contact with, the spherical recess 303a of the lens holder 303.

As shown in FIGS. 23 and 24, the lens holder 314 has a flat side surface(D-cut surface) 314 b at a position proximate to the objective lens 302.The lens holder 314 also has a cutout 314 c formed as a result ofcutting out a part of the spherical surface at a position symmetrical tothe side surface 314 b with respect to the optical axis of the objectivelens 301.

The lens holder 303 includes a projection 303 b around each of theobjective lenses 302 and 301, and a top surface of the projection 303 bis preferably coated with an impact prevention member formed of apolyurethane-based resin. The impact prevention member prevents theobjective lenses 302 and 301 from directly colliding against the opticaldisc (not shown). As shown in FIG. 25, the projection 303 b is formed inan area other than an area facing an outer peripheral area of theoptical disc (not shown).

According to this embodiment, two objective lenses are located adjacentto each other on the lens holder, and the objective lens 301 is securedto the lens holder 303 via the lens holder 314. Because the lens holder314 has the flat side surface 314 b at least at a position proximate tothe objective lens 302, the objective lenses 301 and 302 can be locatedas close as possible to each other.

Since the distance between the two lenses can be decreased for thisreason, no unnecessary space is made in arranging the optical elementsof the optical head. As a result, the optical head can be made compact.

In addition, the two objective lenses are located such that the opticalaxes thereof are close to each other as described in Embodiment 2. Owingto such an arrangement, the interval between the two through-holesacting as the optical paths of the two objective lenses can also be madesmall. This decreases the entire size of the movable body, and alsoreduces the adverse affect of the resonance of the lens holder andrealizes a superb displacement frequency response characteristic. Such asuperb characteristic guarantees a stable servo performance and thusrealizes an optical information apparatus capable of performing a stablerecording or reproduction operation.

As shown in FIG. 25, at least the side surface 314 b of the lens holder314 facing the outer peripheral area of the optical disc 315 is flat.Owing to this, when recording or reproduction is performed on or fromthe optical disc 315 accommodated in a cartridge 316, a cartridge edge316 a facing the outer peripheral area of the optical disc 315 and thelens holder 314 are prevented from contacting each other and thus stablerecording or reproduction is realized.

Similarly, the projection 303 b of the lens holder 303 is formed in anarea other than an area facing the outer peripheral area of the opticaldisc 315. Owing to this, when recording or reproduction is performed onor from the optical disc 315 accommodated in the cartridge 316, thecartridge edge 316 a facing the outer peripheral area of the opticaldisc 315 and the lens holder 314 are prevented from contacting eachother and thus stable recording or reproduction is realized.

As shown in FIGS. 23 and 24, the lens holder 314 has the sphericalslidable surface 314 a and also the cutout 314 c formed by cutting out apart of the spherical surface at a position symmetrical to the flat sidesurface with respect to the optical axis of the objective lens 301.Owing to this, as shown in FIG. 24, the area by which the lens holder314 and the lens holder 303 contact each other can be made symmetricalwith respect to the optical axis. Therefore, when the lens holder 303and the lens holder 314 are bonded together with an adhesive, the amountof deforming by heat and the thermal conductivity are symmetrical, andthe reliability is improved.

As described above, the objective lens driving device according to thisembodiment can be preferably combined with Embodiment 2. The objectivelens driving device according to this embodiment may also be combinedwith Embodiment 1 and Embodiment 2. The objective lens driving deviceaccording to this embodiment may also be combined with Embodiment 1.

Embodiment 4

An optical information apparatus according to an embodiment of thepresent invention will be described with reference to FIG. 26.

An optical information apparatus 104 includes an optical pickup 402, anelectric circuit 403, and a motor 404.

Optical discs 407 through 409 have different recording densities. Theoperator selects one of the optical discs, and places the selectedoptical disc on a turntable 405. The optical disc placed on theturntable 405 is secured thereon by a clamper 406 and is driven torotate by the motor 404.

As the optical pickup 402, the optical pickup 101 described inEmbodiment 1, or the optical pickup including the objective lens drivingdevice 102 or 103 described in Embodiment 2 or 3, can be preferablyused.

The optical pickup 402 is movable in the tracking direction by a drivingmechanism 401 such as a traverse motor or the like and can jump to adesirable track.

The optical pickup 402 outputs a focusing error signal or a trackingerror signal to the electric circuit 403 in correspondence with thepositional relationship with the optical discs 407 through 409. Incorrespondence with such a signal, the electric circuit 403 sends asignal for slightly moving the objective lenses to the optical pickup402. Based on this signal, the optical pickup 402 performs focusingcontrol or tracking control on the optical discs 407 through 409. Thus,the optical information apparatus 104 performs information reproductionor recording.

According to this embodiment, the optical information apparatus includesan optical pickup according to Embodiment 1 or an objective lens drivingdevice according to Embodiment 2 or 3, and so is capable of performingrecording or reproduction on or from a plurality of optical discs havingdifferent recording densities at high precision and stably.

Embodiment 5

With reference to FIG. 27, a computer according to an embodiment of thepresent invention will be described.

A computer 105 includes an optical information apparatus 501, which isthe same as the optical information apparatus 104 described inEmbodiment 4. The computer 105 also includes an input device 503 forinputting information, such as a keyboard, a mouse, a touch panel or thelike, and an arithmetic operation device 502 for performing anarithmetic operation based on information input through the input device503, information read from the optical information apparatus 501 or thelike, such as a central processing unit (CPU) or the like.

The computer 105 further includes an output device 504 for displayinginformation such as an arithmetic operation result provided by thearithmetic operation device 502 or the like, such as a CRT, a liquidcrystal display, a printer or the like.

The computer 105 includes the optical information apparatus 501, whichis the same as the optical information apparatus described in Embodiment4. Therefore, the computer 105 is capable of performing a recording orreproduction operation on or from different types of optical discs; forexample, an operation of recording video information or data or audioinformation or data on different types of optical discs, or an operationof reading such information recorded on different types of optical discsto perform information processing or editing, with high precision andstably.

Embodiment 6

With reference to FIG. 28, an optical disc player according to anembodiment of the present invention will be described.

An optical disc player 106 includes an optical information apparatus601, which is the same as the optical information apparatus 104described in Embodiment 4. The optical disc player 106 also includes aconversion device 602 for converting information obtained from theoptical information apparatus 601 into an image, such as a decoder orthe like. The optical disc player 106 may be used as a car navigationsystem. The optical disc player 106 may also include a display device603 such as a liquid crystal monitor or the like.

Embodiment 7

With reference to FIG. 29, an optical disc recorder according to anembodiment of the present invention will be described.

An optical disc recorder 107 includes an optical information apparatus701, which is the same as the optical information apparatus 104described in Embodiment 4. The optical disc recorder 107 also includes aconversion device 702 for converting image information into informationrecordable on an optical disc by the optical information apparatus 701,such as an encoder or the like. The optical disc recorder 107 may alsoinclude a decoder 703 for converting an information signal obtained fromthe optical information apparatus 701 into an image. The optical discrecorder 107 may further include an output device 704 for displayinginformation, such as a CRT, a liquid crystal display, a printer or thelike.

Embodiment 8

With reference to FIG. 30, an optical disc server according to anembodiment of the present invention will be described.

An optical disc server 108 includes an optical information apparatus801, which is the same as the optical information apparatus 104described in Embodiment 4. The server 108 also include an input device805 for inputting information, such as a keyboard, a mouse, a touchpanel or the like, and a wired or wireless input/output terminal 802 forobtaining information recordable by the optical information apparatus801 or outputting information read by the optical information apparatus801 to an external device. Owing to this, the server 108 acts as anoptical disc server for exchanging information with a network, i.e., aplurality of devices, for example, a computer, a telephone, a TV tuneror the like and sharing information with the plurality of devices. Theoptical disc server 108 may further include an output device 804 fordisplaying information, such as a CRT, a liquid crystal display, aprinter or the like. Where a changer (not shown) for mounting ordismounting a plurality of optical discs on or from the opticalinformation apparatus 801 is provided, the optical disc server 108 canrecord and accumulate a great amount of information.

Embodiment 9

With reference to FIG. 31, a vehicle 109 according to an embodiment ofthe present invention will be described.

The vehicle 109 includes an optical information apparatus 901, which isthe same as the optical information apparatus 104 described inEmbodiment 4.

The vehicle 109 includes a body 915 and a power generation section 912for generating power for driving the body 915. The vehicle 109 alsoincludes a fuel storage section 911 for storing a fuel to be supplied tothe power generation section 912, or/and a power supply 910. Where theoptical information apparatus 901 is mounted on the vehicle 109 havingsuch a structure, the user can obtain information stably from varioustypes of optical discs or record information to various types of opticaldiscs, while being in a movable object. In the case where the vehicle109 is a train or an automobile, the vehicle 109 further includes wheels907 for running and a handle 916 for changing the running direction.

Where the vehicle 109 further includes a changer 902 or an optical discaccommodation section 903, a great number of optical discs can be usedeasily. Where the vehicle 109 includes an arithmetic operation device908 for processing information obtained from an optical disc to providean image, a semiconductor memory 909 for temporarily storing informationor a display device 905, video information can be reproduced from theoptical disc. Where the vehicle 109 includes an amplifier 913 and aspeaker 914, audio information or music can be reproduced from theoptical disc.

Where the vehicle 109 includes a positional sensor such as a GPS 906 orthe like, the current location or the traveling direction can bedisplayed on the display device 905 together with map informationreproduced from the optical disc, and traffic information or navigationinformation can be output from the speaker 914 in the form of a voice.Where the vehicle 109 further includes a wireless communication section904, information can be obtained from outside to be used complementarilywith the information from the optical disc.

In Embodiments 4 through 9, the output device and the input device aredescribed. The devices described in Embodiments 4 through 9 may includeonly an input terminal or an output terminal with no output or inputdevice.

INDUSTRIAL APPLICABILITY

The present invention is preferably usable for an optical informationapparatus for performing at least recording or reproduction on or fromvarious types of optical discs, such as an optical disc apparatus. Thepresent invention is especially preferably usable for an opticalinformation apparatus, such as an optical disc apparatus, including aplurality of objective lenses.

The invention claimed is:
 1. An objective lens unit, comprising: a firstlight source; a second light source; a first objective lens; a secondobjective lens; a lens holder for supporting the first objective lensand the second objective lens; an actuator for driving the lens holder;and a detector; wherein: the lens holder includes a first through-holethrough which light incident on the first objective lens passes and asecond through-hole through which light incident on the second objectivelens passes, light emitted by the first light source is collected on adata recording face of a first optical disc, light emitted by the secondlight source is collected on a data recording face of a second opticaldisc, the first objective lens and the second objective lens arearranged in a direction perpendicular to a tracking direction of theoptical disc, an interval between an optical axis of the first objectivelens and an optical axis of the second objective lens is 2.5 mm orgreater and 5 mm or less, and the actuator for driving the lens holderis provided with at least one electromagnetic driving element on each ofboth end faces of the lens holder, the end faces being located along thedirection in which the first objective lens and the second objectivelens are arranged.
 2. An objective lens unit according to claim 1,wherein the at least one electromagnetic driving element includes atracking coil for driving the lens holder in the tracking direction, andthe tracking coil is a flat coil which is wound around an axisperpendicular to the tracking direction.
 3. An objective lens unitaccording to claim 1, wherein the at least one electromagnetic drivingelement includes a focusing coil for driving the lens holder in afocusing direction, and the focusing coil is a flat coil which is woundaround an axis perpendicular to the tracking direction.
 4. An objectivelens unit according to claim 1, wherein the at least one electromagneticdriving element includes a plurality of focusing coils for driving thelens holder in a focusing direction and a plurality of tracking coilsfor driving the lens holder in the tracking direction, each of theplurality of focusing coils and the plurality of tracking coils is aflat coil which is wound around an axis perpendicular to the trackingdirection.
 5. The optical pickup according to claim 1, wherein the firstobjective lens is used to collect light having a longer wavelength thanlight collected by the second objective lens.
 6. The optical pickupaccording to claim 1, wherein a focal length of the first objective lensis longer than a focal length of the second objective lens.
 7. Anoptical information apparatus, comprising: an optical pickup defined byclaim 1; a motor for driving an optical disc to rotate; and an electriccircuit for controlling the optical pickup based on a signal obtainedfrom at least the first light detector of the optical pickup.
 8. Acomputer comprising the optical information apparatus defined by claim7.
 9. An optical disc player comprising the optical informationapparatus defined by claim
 7. 10. A car navigation system comprising theoptical information apparatus defined by claim
 7. 11. An optical discrecorder comprising the optical information apparatus defined by claim7.
 12. An optical disc server comprising the optical informationapparatus defined by claim
 7. 13. A vehicle comprising the opticalinformation apparatus defined by claim 7.